According to MinnKota's table, the 70-lbf-thrust motor will drive a 20-25
ft, 3500 lb boat, so it would seem to drive a P19. But MinnKota's interest
is strictly in motors for fishing/trolling and not in sailboat auxiliary
power. In smooth water with little wind the motor would probably move and
control the P19 okay, but not as well as a 5 hp outboard. Even at full
power, the MinnKota doesn't seem as powerful as my old 3 hp Seagull, but I
haven't accomplished comparative speed tests and I may be deceived by the
silence of the MinnKota. But 24 volts at 45 amperes equals 1080 watts,
which is equivalent to only about 1-1/2 hp, according to the conversion
factor I have. Obviously you can't get more power out of the motor than
you're putting in, but I don't know what kind of horsepower gas outboards
are rated in. I've only been out with the motor two times and that was with
no other Potters around to compare performance. I would guess that max
power on the MinnKota RT-70S is about equivalent to a 2-1/2 hp outboard.
MinnKota has a Neptune line of motors that they market as primary power
outboards rather than trolling motors, without explaining how they differ
from trolling motors. I E-mailed the MinnKota advisor on their web page and
asked that question, but he apparently hadn't read the MinnKota brochure,
because the only Neptune he was aware of was the one that mounts on the
decavitation plate of a large outboard. He also made the comment that
"MinnKota is in the fishing business." I only mention Neptune because they
have a 96-lbf-thrust model, which is actually two 48-lbf 24-V motors
ganged together, side by side, with a single controller. That might be more
appropriate for a P19. It isn't available in a saltwater model, but
possibly MinnKota would put together a pair of RT-70S motors in the same
fashion. It took 5 or 6 weeks to receive my motor after I ordered it
through West Marine, which made me wonder if MinnKota builds them to order.
You could use two motors but leave one turned off except when you need the
extra push. The RT70S weighs 28 lb.
Your planned battery bank sounds right. I used the 24 series batteries
because of the limited space in the P14. Most #24s are only 85 Ah, but the
Costco Kirkland battery is rated at 95 Ah. The Kirkland is made by Johnson
Controls, one of the three main battery manufacturers. The others are Exide
and Delco. I could add a couple more 24 series batteries in the Potter,
which would bring the total battery weight to 180 lb, equivalent to one
person my size riding in the cabin. The two batteries I've mounted in the
footwell will probably preclude me from sleeping comfortably in the cabin
unless I raise the bunk deck 3 or 4 inches somehow to regain a level
surface.
I love the quiet and cleanliness of the electric motor, and having full
control when approaching a dock is wonderful. With the Seagull, I would
have to shut off the engine and guess at how far I would coast to arrive at
the dock with minimum speed. If there were several boats waiting their turn
to land, I would have to go around in circles instead of hovering.
I checked the charger at 10 pm last night and the batteries were already
fully recharged. Now that I know I can recharge that quickly at the 24 V 10
A setting, I will probably use the 2 A setting if I am not planning to sail
for a couple of days. That will help prolong battery life. The charger also
has a 15 A 24 V setting if I'm really in a hurry. I'm pleased with the
DieHard charger from Sears. I think it was about $90. The charger has a
processor so I don't have to worry about damaging the batteries if I use
the automatic mode. It has settings for 12 or 12 V output, gel cel, deep
cycle, and regular batteries; automatic, manual, or maintenance (storage)
modes of operation; and three charging rates. There is a cheaper 12/24 V
charger (Schumacher, I think) I've seen at Walmart, but it didn't have all
the features of the DieHard.
I would like to find an inexpensive panel-mount digital voltmeter to
monitor the state of charge.
Harry Gordon
P14 #234, Manatee
Mountain View, CA